1. Technical Field
The present invention relates to the cleaning of equipment in which chemical mechanical planarization (xe2x80x9cCMPxe2x80x9d) is performed. In particular, the present invention relates to chemical compositions for cleaning CMP equipment wherein said compositions have improved cleaning performance and/or reduced hazards to the equipment, human staff and environment.
2. Description of Related Art
Planarization is a necessary step in the fabrication of multilayer integrated circuits (xe2x80x9cICsxe2x80x9d), providing a flat, smooth surface that can be patterned and etched with the accuracy required of modem IC components. The conventional planarization technique is CMP (Chemical Mechanical Planarization or Polishing) known in the art and described in text books (for example, xe2x80x9cChemical Mechanical Planarization of Microelectronic Materials,xe2x80x9d by Joseph M. Steigerwald, Shyam P. Murarka and Ronald J. Gutman, 1997). CMP typically makes use of a polishing pad brought into mechanical contact with the wafer to be planarized with an abrasive/reactive slurry interposed between polishing pad and wafer. Typical CMP slurries contain constituents that react chemically with the substrate to be planarized as well as constituents causing planarization by mechanical abrasion. Relative motion of the polishing pad with respect to the wafer leads to polishing of the wafer through mechanical abrasion and chemical etching.
The abrasive slurry polishing materials used in CMP typically comprise an abrasive such as silica, alumina, or ceria and chemically reactive ingredients. Typically, in a practical production environment, these slurry materials cannot be entirely confined to the polishing pad and often splash onto various portions of the CMP apparatus and dry in place leading to increasing deposits of CMP slurry materials at various locations within and on the CMP apparatus itself. Deposits also tend to accumulate on the interior surfaces of delivery tubes and other means used for directing the CMP slurry to the necessary sites. As such deposits build over time, it is a common problem that portions of such slurry deposits break loose in the typical form of abrasive particles that can fall back onto the polishing pads, wafers and/or platens. These unwanted abrasive particles dislodged from slurry deposits on the CMP apparatus are a source of concern to the CMP engineer in that they may cause, inter alia, uncontrollable wafer scratching. Additionally, some slurry polishing materials such as ferric nitrate slurries typically used for CMP of tungsten layers cause unsightly stains on CMP apparatus. Hence, slurry materials and other substances have to be periodically removed from CMP apparatus where they have been deposited. Whether the removal occurs after each pad change, once a week, once a month, or pursuant to any other maintenance schedule does not negate the fact that the apparatus eventually has to be thoroughly cleaned so that slurry and other unwanted extraneous materials are removed therefrom.
Polishing slurries containing ferric nitrate Fe(NO3)3 tend to form ferric hydroxide (Fe(OH)3) and ferric oxide (Fe2O3) residues that tend to precipitate within the conduits of slurry distribution systems that carry slurry to individual CMP apparatus and to specific locations within CMP equipment. These flocculent precipitates can break free, traverse through the slurry distribution system and come into contact with the wafers. Undesirable wafer scratching may result. Hence, there is a need for a cleaning composition effective for removing deposits from the interior regions of the slurry-carrying distribution systems associated with CMP apparatus.
Current practice makes use of surfactant solutions for cleaning CMP apparatus. These solutions are typically not chemically designed to break off or to dissolve typical CMP residues. Thus, the residues must be scraped off with, for example, Teflon scrappers. This scraping technique is both laborious and time-consuming, and results in the generation of loose particles that may fall back onto the apparatus and eventually back onto polishing pads, wafers and/or platen. Uncontrollable wafer scratching and reduced product yields are the typical results. Examples of CMP cleaning systems including wiping operations include those commercially available from The Texwipe Company LLC of Upper Saddle River, N.J. for use in cleaning tungsten CMP slurries (TX8606 SCS) and oxide CMP slurries (TX8065 SCS).
Additionally, current methods for cleaning CMP apparatus make use of hydrogen fluoride (HF) solutions and/or potassium hydroxide (KOH) solutions. Typically, concentrated HF (49%), diluted HF (1% to 10%), or a dilute solution of KOH (e.g., a 5% to 10% solution) is employed. The HF solution or KOH solution is typically sprayed onto the various surfaces of the CMP apparatus that require cleaning and subsequently rinsed with de-ionized (xe2x80x9cDIxe2x80x9d) water. The use of hydrogen fluoride (HF) for cleaning CMP apparatus has serious disadvantages as a result of this substance being extremely deleterious to the human bone structure if absorbed through the skin and as a result of it also being deleterious if inhaled. HF also requires special disposal methods. Thus, stringent precautions must be sustained when working with and disposing of HF. Additionally, HF fails to successfully remove some chemical stains such as ferric nitrate. Furthermore, HF can also damage the platen plates of CMP apparatus.
A multiplicity of disadvantages are also associated with the use of potassium hydroxide (KOH) for cleaning CMP apparatus. For one thing, KOH fails to successfully remove some chemical stains such as ferric nitrate. Additionally, KOH often leaves a residual composition of potassium, a mobile ion, on the apparatus and polishing pad that may contaminate semiconductor wafers thereby resulting in detrimental effects on the electrical performance of the device and a reduction in yield. Furthermore, KOH can also damage the platen plates and some of the material used to form shields of the CMP apparatus (typically Lexan). Although less dangerous to humans than HF, KOH is nevertheless caustic and requires care in handling and disposal.
Thus, there is a need for a cleaning composition for removing unwanted deposits from CMP apparatus that ameliorates or overcomes one or more of the shortcomings of the prior art. The cleaning composition of the present invention is substantially free of HF, KOH. Furthermore, the present invention does not make use of HCl, in contrast to the work of Thurman-Gonzalez et. al. (WO 99/23688).
The present invention relates to chemical compositions and methods of use for cleaning CMP equipment, including the interiors of delivery conduits for carrying CMP slurry to the necessary sites. The chemical compositions of the present invention are also useful for post-CMP cleaning of the wafer itself.
Three classes of cleaning compositions are described, all of which are aqueous solutions. One class operates in a preferable pH range from about 11 to about 12 and preferably contains one or more non-ionic surfactants, one or more simple amines, a surfactant or sticking agent, such as one or more soluble dialcohol organic compounds and one or more quaternary amines. A second class of cleaning composition operates in a preferable pH range of approximately 8.5 and contains one or more of lactic acid, citric acid and oxalic acid. A third class of compositions is acidic, having a preferable pH range from about 1.5 to about 3, preferably containing at least one oxidizing acid, at least one chelating agent, at least one sticking agent and at least one anionic surfactant. HF and KOH are substantially absent from the preferred compositions of the present invention.
Some compositions of the present invention are shown to be advantageously used for cleaning the slurry distribution system of CMP apparatus.
Among the advantages of the present cleaning compositions are those set forth below.
One advantage of the cleaning compositions according to the present invention is that they clean better than cleaning compositions based upon both HF and KOH.
Another advantage of the cleaning compositions according to the present invention is that they are compatible with the typical plastic and metal parts of the CMP apparatus and thus are not as aggressive as HF and KOH in attacking the materials of the CMP apparatus.
A further advantage of the cleaning compositions according to the present invention is that they are far less acidic than HF.
Another advantage of the cleaning compositions according to the present invention is that they ameliorate environmental disposal restrictions related to HF.
A further advantage of the cleaning compositions according to the present invention is that, lacking compounds capable of releasing potassium ions, they do not leave a residual composition of potassium material on the apparatus as occurs with the use of KOH.
Yet another advantage of the cleaning compositions according to the present invention is that they tend to loosen residues thereby reducing the required manual labor. In some cases, the requirement of using a tool such as a spatula for scraping off residues from CMP apparatus is eliminated.
Another advantage of the cleaning compositions according to the present invention is that they typically require merely the use of a sponge, which does not have to be a SCOTCH bright sponge or the like, for removing residues. This tends to eliminate scratches to the CMP apparatus caused by cleaning with much harder and grittier sponges typically used heretofore.
Yet another advantage of the present invention is the ability to clean residue from the interior surfaces of the CMP slurry distribution system.
Another advantage of the present invention is the absence of abrasives.